Liquid crystals are suitable as dielectrics in indicating devices, since their optical properties can be influenced by an electrical potential. Suitable electro-optical devices are well-known to the person skilled in the art. Examples of such devices are cells having dynamic scattering, DAP cells (deformation of aligned phases), TN cells (twisted-nematic) and STN cells (super twisted-nematic) having a twisted nematic structure, guest/host cells, phase change cells having a cholesteric-nematic phase transition and SBE cells (super birefringence effect).
Further, electro-optical devices based on chiral tilted smectic liquid crystals are proposed in Appl. Phys. Lett. 36, 899 (1980) and in Recent Developments in Condensed Matter Physics 4, 309 (1981). In such devices the ferroelectric properties of these materials are used. Suitable tilted smectic phases are, for example, smectic C, F, G, H, I and K phases. There are generally preferred smectic C phases which, in particular, permit high response speeds. The chiral tilted phases are usually denoted by S*.sub.C, S*.sub.F etc., with the asterisk indicating the chirality.
Ferroelectric liquid crystals should have good stability under chemical and thermal influences and in electrical fields. Further, they should have a suitable mesophase over a broad temperature range, low viscosity and especially a sufficiently high spontaneous polarization.
Suitable mixtures used as ferroelectric liquid crystals are one or more optically active doping substances and a liquid crystal material which may include one or more components and which as a rule should have a tilted smectic phase. The optically active doping substances need not themselves be smectic, but in the liquid crystal material they should produce a chiral tilted smectic phase and should induce a high spontaneous polarization. In order to achieve a high spontaneous polarization in the mixture and/or in order that the amount of optically active doping substances can be held relatively low, doping substances which already in small amounts are capable of inducing a high spontaneous polarization are desired.